In some examples, it can be desirable to trigger an action when a time-varying distance reaches a particular threshold. For instance, when an automobile backs up to an object, the automobile can apply the brakes when the automobile reaches a threshold distance from the object. As another example, in an automatic car wash, a cleaning element can approach a surface of the car, but can stop short of touching the surface. For these and other examples, it can be desirable to use a range finder to determine if or when the time-varying distance reaches the threshold.
Some commercially available range finders launch light toward a target, measure the light reflected from the target, and use a time-of-flight of the light to determine the distance to the target. These time-of-flight range finders are inadequate for relatively short distances, such as (but not limited to) 10 meters or less, and can be relatively complex and/or expensive.